Gas‒phase reactions of NO+with Glu and γ‒Glu–Met

The reactivity of the nitrosonium ion, NO+, with the amino acid Glu and the dipeptide γ‒Glu–Met in the gas phase has been investigated using the combination of chemical ionisation mass spectrometry and MS/MS. It is shown that NO+reacts efficiently with both Glu and Glu–Met leading to the formation of the nitroso‒group containing ions atm/z159 and 288.The formation ofm/z159, (GluNO‒18)+, is rationalized by a mechanism involving an electrophilic attack of NO+upon the carbonyl oxygen atom of one of the carboxylic groups of Glu and the N‒terminal carboxylic group of Glu–Met leading to the neutral losses of H2O and Met, respectively. The unimolecular decompositions of the metastable and collisionally activatedm/z159 ions lead primarily to the elimination of the neutral species HNO (major) and the (Glu‒18) residue (minor). The formation of them/z288 ions can be described by a mechanistic scheme which involves the ion‒molecule interaction ofm/z159 with Glu and Glu–Met and subsequent losses of H2O and Met, respectively. Unimolecular and collisionally activated dissociations ofm/z288 suggest the formation of the proton‒bridged ion‒neutral complex [(GluNO‒18)···H+···(Glu‒18)].